Various types of metal contaminants are present in surface water, groundwater, soil, storage tanks, lagoons, industrial gaseous emissions and other sites, often as wastes or byproducts of industrial processes. Arsenic, antimony, beryllium, cadmium, chromium, copper, lead, mercury, iron, manganese, magnesium, radium, nickel, selenium, silver, thallium and zinc are considered to be priority pollutants by the U.S. Environmental Protection Agency (EPA).
Arsenic contamination in surface water, groundwater and soil represents a significant health hazard. Arsenic is used for hardening metals such as copper and lead and as a doping agent in the electronics industry. Arsenic salts are used to make herbicides, rodenticides and fireworks. Arsenic and arsenic compounds are toxic and can be carcinogenic. They are absorbed into the body through gastrointestinal ingestion or inhalation. For example, the trivalent inorganic compounds of arsenic, such as arsenic trichloride, arsenic trioxide and arsine, are highly toxic.
Arsenic-contaminated groundwater has conventionally been treated by groundwater pump and treat technologies including post precipitation, chemical oxidation, filtration, sedimentation, etc. With respect to the treatment of soils contaminated with arsenic, the following methods are currently employed: 1) land farming, where soil piles are watered and aerated; 2) bioreactors that involve the slurry treatment of soil and water in a closed vessel to which oxygen, nutrients and a carbohydrate cosubstrate such as molasses, corn syrup, or hydrolyzed starch are added; and 3) in-situ treatment where contaminated soils are chemically oxidized and/or stabilized using encasement methods.
Contamination of groundwater and surface water by acid mine drainage (AMD) and heavy metals is also a global problem. Disposal of materials such as mine tailing, waste rock and spent oil shale have created severe environmental problems. AMD is contaminated effluent that results from the oxidation of iron-sulfide minerals exposed to air and water. AMD is generated by chemical reactions and bacterial oxidation processes. Sulfide ores contain large quantities of pyrite, which is discarded in the tailings and produces sulfuric acid when exposed to water and oxygen. The ferrous iron produced is then oxidized to ferric ions, which become the dominant oxidizing agent of the exposed sulfide minerals. The reduced sulfur and iron compounds in the deposit provide an environment for T. ferrooxidans which oxidize iron, thiosulfate, sulfur and metallic sulfides to obtain energy for growth while using oxygen as the final electron acceptor and CO2 as its sole source of carbon. This process generates an acidic pH.
AMD resulting from all types of metal mining operations is one of the most pressing environmental problems facing the mining and mineral industries. A significant portion of the AMD draining into rivers and streams is released from waste rock. Once the AMD process has begun, it is extremely difficult to reverse or stop.
Conventional remediation options for groundwater impacted by AMD include preventing the infiltration of contaminants, stabilizing the contaminants chemically, or removal and treatment of the contaminated groundwater. In addition, subaqueous disposal of mine tailings has been employed to avoid terrestrial AMD. However, severe environmental impacts result from subaqueous tailings disposal, including increased turbidity in the receiving waters, sedimentation, toxicity, contamination and fish kills.
Electrolytic plating solutions normally contain high concentrations of heavy metals like zinc, chromium, cadmium, nickel, selenium, copper, gold, silver and nickel. Electroless nickel plating solutions contain a nickel metal salt, such as sulfate, acetate, carbonate or chloride salt, pH adjustors, accelerators, stabilizers, buffers, and wetting agents. The electroless nickel solutions only have a limited useful life and eventually become depleted or spent. The disposal or treatment of spent electrolytic metal plating solutions poses significant challenges for the electroplating industry. The dissolved metal concentration must be below discharge thresholds in order to allow for the solution to be discharged as non-toxic waste directly to a municipal wastewater treatment facility. The spent solutions from the electrolytic and electroless plating processes pose a severe hazard to the environment, if disposed of improperly, and a high monetary cost, if disposed of properly.
A number of wastewater treatment processes have been developed to reduce the metal content in spent plating solutions to low levels prior to discharge. Many current methods involve the removal of dissolved metal from solution by chemical reduction. The spent electroless solution is first contacted with a reducing agent for sufficient time to cause the dissolved metal salt to undergo chemical reduction, resulting in the precipitation of the metal compounds out of the solution. Some methods include the dosing of electroless baths with caustic soda to precipitate the bulk of the heavy metal contaminants as insoluble hydrous oxides (metal hydroxides), pressing the sludge into a filter cake, drumming and disposal. Another waste treatment used for spent electroless plating solutions is the dosing of the solution at slightly alkaline pH with reducing agents. The reducing agents typically used to convert the dissolved metal salt into insoluble metal precipitates include sodium borohydride, sodium hydrosulfite and other chemicals. A further waste treatment method known for reducing the dissolved metal content of spent electroless baths to acceptable discharge levels involves organosulfur precipitation of the metal by dosing the spent solution at a pH of 5-8 with water-soluble precipitating agents.
The bioremediation of various pollutants with butane-utilizing bacteria is disclosed in U.S. Pat. Nos. 5,888,396, 6,051,130, 6,110,372, 6,156,203, 6,210,579, 6,245,235 and 6,244,346, each of which is incorporated herein by reference.